The present invention relates to an electrochemical cell according to the precharacterising clause of Claim 1.
An SOFC cell of this type is disclosed in Japanese Patent publication 10 021931 A. In this publication an anode is shown wherein a first layer consists of metallic particles on which very fine oxide particles have been deposited. The second layer, which is located further away from the electrolyte, consists of a mixture of metallic particles and coarser ceramic particles. These ceramic particles are smaller than the metallic particles. In International Application PCT/NL 93/00256 in the name of Stichting Energieonderzoek Centrum Nederland (ECN) it is described that for SOFC cells it is desirable to have an anode layer with which both the electrochemical activity and the conduction of electrons are optimised. The electrochemical activity is, in particular, determined in the part that is directly in contact with the electrolyte, whilst it is obvious that conduction of electrons is essential for functioning of the cell. Ultimately, these electrons move towards the current collector of the anode. To provide these characteristics it is proposed to make up the anode from a mixture which after sintering and reduction consists of relatively small metallic particles for conduction of electrons and the electrocatalytic activity and oxides for mechanical stabilisation of the anode and matching of the coefficient of thermal expansion to the electrolyte. Close to the interface with the electrolyte, the metallic particles act as a catalyst to promote the electrochemical reaction. According to the abovementioned European application, the particle size of the various components is so chosen that after sintering the metal particles are smaller than the oxide particles in order thus to ensure adequate conduction of electrons.
It has been indicated above that the various particles have different functions depending on the position in the anode. If, for example, nickel is used as the metal for the particles, this has mainly an electrocatalytic function close to the interface with the electrolyte, whilst closer to the current collector the conduction of electrons becomes more important. The same applies for the oxides. The latter must display oxygen ion conduction, in particular close to the interface with the electrolyte, whilst closer to the current collector it is important that there are adequate possibilities for the metal mixed with the oxide particles to be able to provide current-conducting paths. The construction according to Japanese Patent 10 021931 mentioned above meets these requirements only partially. Close to the current collector the network is relatively weak because of the relatively small oxide particles compared with the metal particles. As a result it is not possible to guarantee metallic contact between the metal particles in the longer term and electron conductivity will become inadequate in the long term.
The aim of the present application is to provide an anode for an electrochemical cell, and more particularly an SOFC cell, with which the requirements imposed depending on the position in the layer can be met.
This aim is achieved with an SOFC cell as described above having the characterising measures of Claim 1. Because the oxide particles close to the current collector are larger than the metal particles, an optimum metal network is provided. As a result optimum conduction of electrons can be guaranteed even in the long term. According to the invention, an electrochemical cell which has high stability as a result of a stable metal network is obtained.
According to an advantageous embodiment of the invention, the average particle size of the oxides is  less than 1 xcexcm close to the electrolyte and  greater than 2 xcexcm close to the current collector.
In this context the average diameter of the metallic particles and in particular of nickel is in particular between 1 and 2 xcexcm.
According to the invention small oxygen-ion-conducting oxide particles are used close to the electrolyte. Examples thereof are ion-conducting oxides of the crystal structure class of fluorites or perovskites, and in the case of fluorites zirconia, cerium and hafnia doped with trivalent rare earth metal ions or divalent alkaline earth metal ions and, in the case of perovskites, ion-conducting zirconates, cerates and gallates. Closer to the current collector clear paths along which the electrons move will be produced because of the discrete distance between the various grid components of said current collector. The most important function of that part of the anode that faces away from the electrolyte and is in contact with the current collector is that of a current-collecting layer and, according to the invention, this part is formed by a cermet of coarse oxide particles (alumina, YSZ, GCO, perovskites) and small electron-conducting metal particles. As a result of the relatively coarse oxide particles, nickel paths are produced when nickel is used. It will be understood that other metals known from the prior art, such as copper, other semi-noble metals and noble metals, can be used instead of nickel.
The range of the coarse oxide particles, that is to say the oxide particles which are closest to the current collector, is preferably between 2 and 15 xcexcm. That of the fine oxide particles, that is to say the particles which provide for (oxygen) ion conduction, is preferably between 10 nm and 1 xcexcm.
Such an anode can be produced in any conceivable manner. A particularly simple method is to build it up layer by layer. With this method a first layer is provided which contains the relatively fine oxide particles and is intended subsequently to be placed in contact with the electrolyte, and a second layer is provided which contains the relatively coarse oxide particles. Such a double-layer anode can be produced by any method known from the prior art. Tape casting is a generally known technique for the production of anodes and can be used particularly advantageously for such double-layer anodes. Another technique is screen printing. Optionally the electrolyte is produced at the same time with the aid of these techniques. With such a method in general the metal or metal mixture will be present in the form of metal oxides and when the SOFC cell is started up the metal oxides will be converted to metals at 600-1000xc2x0 C.
It has been found that, compared with conventional anodes, an anode built up in this way has improved characteristics, measured as electrochemical performance, durability and, when used as a reformer, for example when natural gas is used, the methane-water vapour reform rate, that is to say the conversion to hydrogen, is increased.
It must be understood that the coarse and fine oxide particles can be either the same or different. After all, no or less stringent requirements with regard to the oxygen-ion-conducting character thereof are imposed on the coarse oxide particles close to the current collector. The main important aspect is that said coarse oxide particles in combination with the metal particles found in this part of the anode are capable of providing electron-conducting paths. As a result it is possible to use relatively inexpensive material, such as alumina, for the coarse oxide particles. In principle the same applies in respect of the metals used when the anode is built up of various layers.